1. Field of the Invention
This invention relates to a method of forming cellulose paper based honeycomb structures.
2. Description of Related Art
Honeycomb structures, i.e. structures with regularly shaped (usually hexagonal) cells, may be manufactured, usually in the form of panels, from a variety of materials such as metal, aramid or cellulose paper. U.S. Pat. No. 6,194,477 B1 (European Patent Application 0967070A) describes a method of making a cellular material in which an expanded cellular (honeycomb) structure is made from a dense, non-porous cellulose paper, which paper has an air permeance of less than 30 ml/min. An aqueous composition is applied to the cellular structure which is then heated sufficiently to stabilize the structure for commercial stability. The resulting cellular structure is coated with a thermosetting resin and the resin is then cured.
Aramid based honeycombs tend to comprise NOMEX(copyright), that is poly(m-phenylene isophthalamide), which is made by DuPont (Wilmington, Del.).
It is commonplace to undercure the resin coating of such honeycombs in order to achieve improved formability of the products. This gives rise to a higher moisture content than is desirable thus compromising the structural integrity of the product.
In order to form the NOMEX(copyright) based panels into a desired shape it is first necessary to heat the panels in excess of 200xc2x0 C. before forming the panels into shape on cooling. Because of the cellular configuration of the core, the nature of the material from which it is made and the inefficiency of convection heating, the NOMEX(copyright) core heats up relatively slowly and unevenly in the convection oven. The difficulty in evenly heating the NOMEX(copyright) core also places significant constraints on the uses of the formed core. Even when the formed NOMEX(copyright) core is of acceptable quality it cannot generally be used in load-bearing applications because its strength is reduced by charring and other degrading effects of convection heating. Moreover, the final configurations which may be obtained are limited. For example, curved configurations are limited to relatively low degrees of curvature. U.S. Pat. No. 5,119,535 describes the use of a fluidized bed to provide relatively even heating of NOMEX(copyright) core. The fluidized bed is heated to a temperature sufficiently high to soften the NOMEX(copyright) core and sufficiently low to avoid damaging the core. The core is immersed in the fluid and pressure is then applied to form the core against a mold. This is preferably done after the core and mold have been removed from the fluid. The core is then allowed to cool and harden. This method allows the core to be formed into more complex configurations than was previously thought possible. For example, an eight-pound core (123 Kg/m3) with a thickness of up to four inches was allegedly successfully formed into a 90xc2x0 configuration at 400xc2x0 to 600xc2x0 F. However, it is noted that this forming method is expensive to carry out in that it involves heating to high temperatures. It is difficult to assess the required duration of the heating step and as excess heating leads to core degradation the process could lead to significant failure unless carried out by an experienced operator. NOMEX(copyright) core is an expensive material and consequently such waste is unacceptable in commercial applications.
The cellulose based structures made in accordance with the specification of U.S. Pat. No. 6,194,477 B1 (European Patent Application 0967070A) are sold as flat (i.e., planar) sheets only. It has been found by the applicants of the present application that such panels may be formed if heated to a temperature above 200xc2x0 C. However, this heating degrades the cellulose paper and consequently the mechanical performance of the formed honeycomb is dramatically adversely affected by the heating process.
The present invention seeks to provide a method of successfully forming products having cellulose-based substrates, such as those made in accordance with the method disclosed in U.S. Pat. No. 6,194,477 B1 (European Patent Application 0967070A), without significantly adversely affecting the mechanical properties of the honeycomb. For the avoidance of doubt by xe2x80x9cforming the materialxe2x80x9d we mean altering the shape of the material.
According to the present invention there is provided a method of forming a cellular material by facilitating sufficient moisture pick up by the cellular material to enable forming of the cellular material into a final desired shape and subsequently removing sufficient water from the cellular material such that the cellular material then retains the said shape, the walls of the cells of said cellular material comprising a dense, non-porous cellulose based paper.
The aforesaid cellulose based paper preferably has an air permeance (before being formed into cells) of less than 30 ml/min.
The above discussed and many other features and attendant advantages of the present invention will become better understood by reference to the detailed description when taken in conjunction with the accompanying drawings.